The present invention relates to devices which are capable of (1) exhibiting decreased electrical resistance or (2) generating a current or voltage when exposed to electromagnetic radiation or (3) emitting electromagnetic radiation when electrical current is passed. Theses types of devices when exposed to radiation of suitable wavelength, generate electric current or voltage. When electric current is passed through them, they generate radiation of infra-red or shorter wavelengths depending on the value of the semiconducting energy gap. Iron disilicide now appears to be the one of the above-listed metal silicide radiation sources that exhibits the best performance. Infra-red radiation emitted by these radiation sources ranges in wavelengths from 0.77 microns to 1000 microns.
There are numerous applications for infra-red detectors; one is for terrestrial imaging from space. The limited wavelengths which can be transmitted through the atmosphere are approximately 1.5 to 1.9; 2.0 to 2.6; 3.4 to 4.5 to 5.0 and 8 to 13 microns. NASA has shown an interest in the 2.5 to 30 micron wavelength range. Another application for the present invention is in combination with fiber optic systems using silica based fibers (which in long haul, high capacity systems have narrow spectral windows centered on about 1.3 and 1.55 microns). A short haul system has an additional spectral window from about 0.8 to 0.9 microns as well as the windows at about 1.3 and 1.55 microns. In such applications, the output of the infra-red sources can be applied directly to the fiber optics for transmission to an infra-red detector and its associated processor. Since the intrinsic electromagnetic radiation detector and source devices are silicon-compatible, they can be combined on the same chip as other silicon based elements such as data storage and data processing elements. In such a combination, the signal processing and related computing can be performed on the very same chip that holds the source, detector, imaging or detector array. Monolithic systems afford many advantages compared to hybrid systems.
The detectors can be arranged singly or in an array. A two dimensional array can be constructed. Each element in the array has an output which can converted into a digital electrical signal.
Practical devices currently available include intrinsic infra-red semiconductor detectors as discrete devices or linked to electronic circuitry in some form other than on a single silicon chip. Schottky barrier infra-red detectors are also available and workable but are slow for communication purposes and have relatively low quantum efficiency compared to the devices of the present invention. The Schottky barrier are of limited wavelength range. They have been integrated on a silicon chip.
Silicon intrinsic detectors are effective for visible light and perhaps can be extended in time to wavelengths up to about 0.9 microns. Extrinsic silicon detectors are sensitive to much longer wavelengths, but have absorptive coefficients of 1000 to 10,000 times lower than those of intrinsic detectors.
Germanium and germanium-silicon alloys can be grown on a silicon wafer. The absolute long-wavelength limit for germanium based alloys is 1.0 microns and virtually pure germanium has a value of about 1.9 microns. However, germanium and germanium-silicon alloys are relatively weak absorbers of infra-red radiation compared to the transition metal silicide semiconductors described herein. Special structures, such as wave guides, must be developed to use both germanium and germanium-silicon alloys as thin films. The wave guides and other structures are necessary because such devices are weak absorbers of infra-red radiation.
There is also available a family of Mercury-Cadmium-Tellerium devices for infra-red detection. These devices operate without being able to be combined, to date, with an effective microelectronics technology as is possible with silicon based devices.
The devices described above have been effective to some extent. However, there still remains a need for detectors meeting all of the following characteristics:
(1) The efficiency of an intrinsic semiconductor detector;
(2) Efficient operation in the previously described spectral ranges; and,
(3) Practical fabrication on a silicon chip in a monolithic structure. The need for such devices has been recognized by persons skilled in this art and some attempts have been made recently to fabricate such a device using gallium arsenide (GaAs) and related compounds on a silicon substrate. However, these materials are not currently compatible with silicon processing.
The semiconducting metal silicide devices meet all of these criteria for radiation detectors and radiation sources. There also remains a need for radiation sources which exhibit high efficiencies and may be practically fabricated on a silicon ship in a monolithic structure. Again, GaAs and related compounds are under investigation. However, such materials are not currently compatible with silicon processing.